The present invention relates to a casting die.
To save weight, cylinder block/crankcase units are increasingly being manufactured from aluminum alloys by a variety of casting methods, preferably die-casting. Since aluminum alloys, which are good for casting, often do not meet the tribological requirements along the bearing surfaces of the cylinders, measures are taken to improve the properties of the material in these areas. One of these measures is the casting in of cylinder liners.
German Published Patent Application No. 44 38 550 describes a crankcase with cylinder liners made of hypereutectic aluminum-silicon alloys. The alloys described are particularly wear-resistant due to their high silicon content. Moreover, cylinder liners of this kind have a low relative density andxe2x80x94a particular advantage in comparison with cylinder liners based on ironxe2x80x94their thermal expansion coefficient is closer to that of the aluminum casting alloy than the expansion coefficient of iron.
Irrespective of the type of liners, bonding between the solidified casting metal (surrounding cast metal) and the cylinder liner is often inadequate. This results in gaps, which hinder heat transfer between the cylinder liner and the surrounding cast metal. In many cases, bonding between the liner and the surrounding cast metal is better toward the bottom of the liner, toward a crankshaft space, than toward the top, close to the cylinder head. However, it is precisely in the vicinity of the cylinder head that the thermal and mechanical loads on the cylinder liner are greatest. In the case of conventional engines, the slight bonding between the cylinder liner and the surrounding cast metal does not result in any functional disadvantage. In the case of modern engines, which feature very high-pressure forced induction, improved bonding between the liner and the surrounding cast metal may be necessary.
It is an object of the present invention to improve the bonding between a cylinder liner and the surrounding cast metal in a cylinder block/crankcase, especially on the cylinder-head side.
The above and other beneficial objects of the present invention are achieved by providing a casting die as described herein.
The casting die according to an example embodiment of the present invention corresponds to a conventional casting die (in particular a die-casting die) for cylinder block/crankcase units to the extent that it is constructed from at least two die parts which, when closed, form a die cavity in the form of the crankcase. For ejection of the cavities and the bore, the die generally has a plurality of slides. To produce the cylinder bores, the casting die includes cylindrical sleeves. The sleeves may be part of a slide (sleeve slide, in the case of V-engines, for example) or may be secured rigidly in the die cavity (in the case of in-line engines, for example). The casting die is generally a die-casting die but other casting methods, e.g., methods similar to die casting, may be employed in accordance with the present invention.
The sleeves extend through the casting die from a wall on the cylinder-head side to a wall on the crankshaft side (crank space slide). The wall on the cylinder-head side of the casting die represents the surface of the subsequent crankcase on which the cylinder head is mounted after the machining of the parting plane of the cylinder head.
Cylinder liners are placed on the sleeves and subsequently at least partially form the bearing surfaces of the cylinders. The cylinder liners are fixed on the sleeve so that the cylinder liner ends, e.g., at least 3 mm before the wall on the cylinder-head side. The cylinder liner is held at least this distance by a spacer in the casting die.
One advantage of this arrangement may be that a melt of a casting metal (e.g., an aluminum alloy but also a magnesium alloy) may flow past an upper edge of the cylinder lining during filling and overflows the latter. Accordingly, the melt has a relative velocity with respect to the upper edge of the liner, contributing to a surface layer on the liner consisting essentially of an oxide skin being torn open and removed.
The oxide skin, which occurs on any light-alloy surface and thus also occurs on the cylinder liners, which may contain aluminum, prevents optimum joining of the solidified casting metal (surrounding cast metal) and the cylinder liner. Once this oxide skin (which may also contain other, organic impurities, e.g., soot, dust, residues of the die release agent, etc.) has been removed, the melt may locally melt the surface of the liner and join cohesively to it. A cohesive joint between the cylinder liner and the surrounding cast metal may be advantageous particularly in the region of the liner close to the cylinder head since it is here that the highest pressures and thus the highest mechanical loading may act on the liner. The casting die according to the present invention may result in a cylinder block/crankcase which may withstand higher pressures and thus may allow a higher engine power.
The spacer may be in each case mounted on the sleeve on which the cylinder liner is placed. The spacer on the sleeve may be in the form of an offset. The offset extends radially around the sleeve and the upper edge of the liner rests at least partially on the offset.
The offset may be interrupted one or more times as it extends around. This also includes an offset that is in the form of a single nose or of the plurality of noses.
By its very nature, the offset produces a recess in the cylinder track in the cast crankcase above the cylinder liner. Where the overcast region above the cylinder liner is retained when the engine is assembled, it is possible to configure the offset with a limited radial depth. The radial depth of the offset may be less than the radial removal of material during the finish machining of the bearing surface of the cylinder.
The spacer may also be part of the cylinder liner, e.g., in the form of pinnacles or peaks, e.g., through an encircling offset on an upper edge of the cylinder liner. The spacer may also be formed by an additionally inserted distance ring.
Above the respective cylinder liner, the casting die may have an opening leading to a cavity, into which melt may flow off. The effect of this arrangement may be that the melt has a relative velocity with respect to the liner in the upper region of the liner since it may flow off into the cavity. The kinetic energy of the moving melt may break up the oxide skin on the liner.
The cavity may be connected to the opening by a narrow passage, which may be cut off with little effort after the solidification of the component. The cavity and the passage may be arranged in a parting plane of the die to ensure a good casting ejection capability.
The cylinder liner may be composed of a hypereutectic aluminum-silicon alloy. Compared with aluminum casting alloys, such alloys may have improved wear resistance and may be distinguished by their low relative density and similar thermal expansion coefficients to those of the aluminum casting alloy. However, all materials with good wear characteristics may be suitable as a material for cylinder liners, including, e.g., ferrous materials.
Example embodiments of the present invention are explained below with reference to the figures.